As known to all, carotenoid is a kind of very important substance. Carotenoid is often composed through coupling of molecules of five carbon atoms, namely isoprene, and there are multiple unsaturated bonds in the molecular structure. The existence of these conjugated double bonds has endowed carotenoid with related color and some important physiological functions. While based on whether or not oxygen exists in the molecular structure, carotenoid can be further divided into two types, carotenes and xanthophyll, respectively, the former including α-, β-, γ-carotenes, and lycopene, etc, while the latter covers lutein, zeaxanthin, astaxanthin, and canthaxanthin, etc. These carotenoids are all biochemically and commercially quite important, and all have corresponding important physiological functions. The lutein and zeaxanthin of them have aroused increasing recognition from people in recent years, and new progresses have been made continually on research and product development related in this area.
The structural formulas of lutein and zeaxanthin are respectively as follows. They are isomers and the only difference between the two structural formulas is the positions of double bonds on one (instead of two) end ring. The positions of double bonds on two end rings for the former are symmetrical, while those for the latter are asymmetrical. The entire linear chain part for each molecule of lutein and zeaxanthin is a conjugated structure, namely, provided with alternate double bonds and single bond. In the molecule of zeaxanthin, the conjugated structure extends to the first bond on the two end rings, while the conjugated degree for lutein is even lower, as the correct arrangement of complete conjugated structure has not yet been formed for double bonds on one of its end rings. This is the discrepancy in molecular structure that has caused certain difference in the functions of lutein and zeaxanthin.

As natural pigment matters lutein and zeaxanthin have a very wide distribution in natural world. They mainly exist in higher plants, alga, fishes, shells, and bacteria, and they always exist inside organisms in the form of esters. In these matters, marigold is a good source of lutein and zeaxanthin, and there is often about 2 g xanthophylls in 100 g marigold fresh flowers, mainly lutein wherein, accounting for more than 90%, while the rest are zeaxanthin and a few other carotenoids. The same as in marigold, among sources from other higher plants and alga, lutein has accounted a larger proportion relative to zeaxanthin, while the amount of zeaxanthin is larger than that of lutein in corn.
Additionally, it has also indicated in molecular structure that stereo isomerism exists in lutein and zeaxanthin, and the structure of stereo isomerism varies along with different sources, as in plant raw material (3R,3′R,6′R)-lutein or (3R,3′R)-zeaxanthin are the main forms, but in animal sources such as fishes and shells, lutein exists in the (3R,3′R,6′R)—, (3R,3′R,6′S)—, and (3R,3′S,6′S)— forms, while zeaxanthin exists in the (3R,3′S)— and (3S,3′S)— forms.
As a kind of carotenoid, lutein and zeaxanthin are initially and now still used as a pigment, such as used in the pigmention of egg yolk, the out skins of poultry (such as skin, leg, and beak) and hypodermal fat, muscle, as well as the outer skins of fishes and shells (skin, scale, and shell). Also it has recently begun to use lutein and zeaxanthin as a functional food colorant. However, as mentioned afore, as there is one extra conjugated double bond in the molecular structure of zeaxanthin than that of lutein, zeaxanthin can show stronger golden yellow color and more lasting color than lutein, and on this account, zeaxanthin is preferred for use under many conditions.
Besides, lutein and zeaxanthin are the only two carotenoids existing in the retina of human body, and they are very important for curing and preventing age-related macular degeneration (ARMD) disease, able to effective protect eyes from suffering ARMD and blindness therefrom. Since zeaxanthin is fully conjugated, it is able to certain degree to provide better protective function for trauma arising from resistance to luminous energy.
In fact, some researches in the mid and late stages of the 80s have also proved that zeaxanthin exists mainly in the subzone at the positive macular center of human eyes, and the amount of zeaxanthin would gradually reduce deviating concentrically from the sunken part and approaching the outer macular circumference, while that of lutein will gradually increase alongside. In the macular periphery, lutein is the main yellow coloring matter.
As mentioned afore, lutein and zeaxanthin is a kind of important colorant and component with unique physiological activity. However, as animal bodies themselves are not able to synthesize carotenoid, they must digest lutein and zeaxanthin from plant sources. There is relatively abundant lutein in the source of plants, and it has been achieved to extract lutein from plants on a large scale, while it is evidently unrealistic to extract zeaxanthin which has the same and even superior efficacy as lutein from plants. Thus, it is necessary to get high-content edible zeaxanthin suitable to human beings through other approaches.
Talking about existing techniques, preparation of zeaxanthin can be basically divided into 3 types—fermentation method, complete chemical synthesis method, and transposition from lutein. The microorganisms used for fermentation method are mainly cells of Flavobacterium. The defect with zeaxanthin produced by fermentation method is the output of most microorganisms is rather low, and sometimes there is a high content of needless or possibly harmful S—S configuration and meso isomers in the fermented products. Moreover, fermented products shall pass through a rather tanglesome follow-up extraction process. The severe disadvantage with complete chemical synthesis method is that they usually need many reaction steps, with a low yield of final products, and during chemical synthesis course, it may generate more needless S—S and S—R stereoisomers of zeaxanthin, in addition to various transformed and degraded products. From a comparison with the two abovementioned methods, it may be a promising method to produce zeaxanthin with lutein transposition method, as only one step reaction is involved in this process. Under the circumstance when reaction condition is well controlled, the product yield will be rather high, and only one kind of R—R isomer of zeaxanthin may exist in the product, with comparatively higher physiological activity.
The patent of CN1082507C has disclosed a method for making zeaxanthin through chemical transposition using lutein as raw material, where mainly the mixture of dimethyl sulfoxide or with saturated alkane and/or arene organic solvent is used as solvent, and hydroxide of alkali metals is used as catalyst to produce zeaxanthin through transposition of lutein. However, in this process, the dosage of alkali is quite great, reaching 250-500 times (mol ratio) that of the lutein raw material. Under the effect of strong alkali at this high concentration, high temperature (80-100° C.) and long duration (reaction time reaching tens of hours), it may cause quite a large part of lutein and the generated zeaxanthin to be degraded or carbonized, though the proportion of zeaxanthin against total carotenoid in the final product after refined crystal with CH2Cl2/CH3OH can reach 90% or so, the yield of products is quite low (The repeated tests have indicated that the product yield by this method is less than 30%), and therefore it is not suitable for industrialized production. When catalysts for phase transfer are used, the treatment process of extraction, column chromatography or re-crystallized purification is specially required, to ensure the product purity. Moreover, hexane, heptane, dichloromethane, methanol, and other organic solvents have been used in reaction process, while it is evidently improper to produce food-grade or medicine-grade zeaxanthin using these toxic solvents.